1. Field of the Invention
The present invention relates to a nitride-based semiconductor light-emitting device, and more particularly, it relates to a nitride-based semiconductor light-emitting device having nitride-based semiconductor layers formed on a substrate.
2. Description of the Background Art
A nitride-based semiconductor light-emitting device such as a nitride-based semiconductor light-emitting diode (LED) or a nitride-based semiconductor laser diode (LD) consisting of InXAlYGa1-X-YN (0≦X, 0≦Y, X+Y≦1) has recently been put into practice.
A conventional nitride-based semiconductor light-emitting device basically has a double heterostructure obtained by successively stacking an n-type nitride-based semiconductor layer consisting of n-type AlYGa1-YN (0≦Y≦1), an active layer consisting of InXGa1-XN (0≦X≦1) and a p-type nitride-based semiconductor layer consisting of p-type AlZGa1-ZN (0≦Z≦1) on a substrate. In general, the nitride-based semiconductor light-emitting device further comprises an n-type contact layer for implementing ohmic contact with an n-side electrode and a p-type contact layer for implementing ohmic contact with a p-side electrode. A conventional nitride-based semiconductor laser diode may have an n-type optical guide layer and a p-type optical guide layer formed to hold an active layer therebetween.
The n- or p-type nitride-based semiconductor layer of the aforementioned nitride-based semiconductor light-emitting device is prepared by doping a nitride-based semiconductor with dopants providing n-type carriers (electrons) or p-type carriers (holes). In order to obtain a nitride-based semiconductor light-emitting device having excellent luminous efficiency, it is inevitably necessary to suppress light absorption in each nitride-based semiconductor layer. However, the activation efficiency of the dopants for the p-type nitride-based semiconductor is so low that the dopants must be doped in a large quantity in order to obtain a p-type nitride-based semiconductor having prescribed carrier concentration in general. In this case, light absorption is inconveniently increased in a p-type contact layer or a p-type optical guide layer having a small band gap due to dopant levels resulting from introduction of the large quantity of dopant in the p-type nitride-based semiconductor. The light absorption is further increased also by crystal defects resulting from the large quantity of dopant.
In general, therefore, there is proposed a nitride-based semiconductor laser diode capable of reducing light absorption resulting from the dopants by forming an undoped optical guide layer on an active layer in place of the p-type optical guide layer, as disclosed in “Technical Report of IEICE”, the Institute of Electronics, Information and Communication Engineers, Jun. 15, 2002, pp. 63-66, for example.
However, the aforementioned proposed conventional nitride-based semiconductor laser diode has no countermeasure for preventing a p-type contact layer from light absorption. The p-type contact layer is doped with a large quantity of dopant for implementing ohmic contact with a p-side electrode. Also when light absorption is reduced in the aforementioned undoped optical guide layer, therefore, it is difficult to inhibit the p-type contact layer from light absorption resulting from the dopants. Consequently, it is disadvantageously difficult to improve luminous efficiency of the nitride-based semiconductor laser diode. In a nitride-based semiconductor light-emitting diode emitting light through a p-type contact layer, influence exerted on emission characteristics of the nitride-based semiconductor light-emitting device is disadvantageously increased if light absorption is increased in the p-type contact layer.
On the other hand, there is also proposed a technique of forming a contact layer having a modulation doped superlattice structure by alternately stacking undoped nitride-based semiconductor layers and p-type nitride-based semiconductor layers doped with dopants, as disclosed in Japanese Patent Laying-Open No. 2001-60720, for example.
In the aforementioned technique disclosed in Japanese Patent Laying-Open No. 2001-60720, however, the dopants may diffuse into the undoped nitride-based semiconductor layer from the nitride-based semiconductor layers doped with the dopants arranged on both sides (upper and lower sides) of the undoped nitride-based semiconductor layer in the contact layer. In this case, it is difficult to inhibit the contact layer from light absorption due to dopant levels formed in the intentionally undoped nitride-based semiconductor layer. Consequently, it is disadvantageously difficult to improve luminous efficiency of the nitride-based semiconductor light-emitting device despite the modulation doped superlattice structure of the contact layer formed by stacking the undoped nitride-based semiconductor layer and the p-type nitride-based semiconductor layers doped with the dopants.
In order to inhibit a p-type contact layer from light absorption, a p-type nitride-based semiconductor layer having a large band gap may be employed as the p-type contact layer thereby suppressing light absorption. When the band gap of the p-type contact layer is increased, however, a barrier at the interface between the p-type contact layer and a p-side electrode is so increased that it is difficult to implement excellent ohmic contact between the p-type contact layer and the p-side electrode. Thus, the nitride-based semiconductor light-emitting device is disadvantageously reduced in luminous efficiency and increased in operation voltage.